This invention relates to quartz glass elements having improved thermal dimensional stability making them suitable for use at elevated temperatures for extended periods of time. More particularly, this invention relates to improved quartz glass elements having a hollow or tubular shape especially useful for producing semiconductor elements at temperatures higher than previously attainable with prior quartz glass elements.
In the art of producing semiconductor elements such as diodes, four-layer diodes, transistors, integrated circuits and the like, diffusion processes are employed, for example, for doping the semiconductors. For this purpose, the semiconducting crystal is subjected at high temperatures to the action of different gaseous atmospheres, for example, a phosphorus atmosphere and/or a gallium atmosphere. The semiconducting crystal or crystals which in many cases have the shape of small plates are then secured on a carrier of quartz glass. For carrying out the diffusion process, this carrier upon which the semiconducting crystals are applied is inserted into a diffusion tube of quartz glass in an electrically heated annealing furnace. The gaseous atmosphere which is predetermined for doping the semiconducting crystals is then maintained in the diffusion tube at a predetermined diffusion temperature. This is usually done by conducting the doping substance through the quartz-glass tube in the form of a gaseous current.
In order to carry out their functions properly, the semiconducting elements must possess certain properties, especially insofar as the cross-sectional diffusing shape, the resistance values and the durability of the carriers are concerned. These properties are, however, determined to a very considerable extent by the amount of impurities and also by the presence of so-called semiconductor poisons which might enter and contaminate the semiconducting elements during the course of their production. Therefore, not only the raw material but also all other materials which might possibly affect the purity of these elements in the course of their production either directly or indirectly have to comply with extremely high requirements of purity. When quartz glass is employed as a material in the production of semiconducting elements, these requirements previously necessitated the carriers for the semiconducting crystals and the diffusion tubes to be made of a quartz glass of such a high degree of purity that it preferably contained a total of less than 4 ppm (parts per million) of metallic impurities.
In order for the diffusion treatment to be carried out within the shortest possible length of time, it should be effected at the highest possible temperature since the rate of speed of diffusion increaases very rapidly as the temperature is increased. However, the upper temperature limit at which diffusion treatments could previously be carried out in actual practice amounted only to approximately 1,200.degree. to 1,280.degree. C. and often they had to be carried out at a temperature of less than 1,200.degree. C. since by remaining continuously in the annealing furnace the diffusion tubes of quartz glass were often plastically deformed to the extent that the carriers with the semiconducting crystals thereon would no longer fit into the tubes.
The French Pat. No. 1,293,554 discloses a diffusion tube which consists of quartz and is provided on its outer side with a coating which will become liquid at the temperature at which the semiconducting crystals are treated. This coating is intended to prevent impurities from penetrating by diffusion through the quartz diffusion tube into the area within the quartz tube which forms the treating chamber for the semiconducting crystals. However, such diffusion tubes of quartz glass which remain continuously in the annealing furnace also have the disadvantage that they will be plastically deformed very considerably when the diffusion temperature is made too high.
The deterioration of diffusion tubes by cracking is caused by irregular devitrification or recrystallization of the quartz glass brought on by exposing such tubes to elevated temperatures such as those encountered in preparing semiconductor elements. Prior attempts to overcome this problem of thermal instability were directed at preventing or retarding devitrification or recrystallization. For example, in U.S. Pat. No. 2,904,713, quartz glass is produced wherein substantially no crystallization seeds are present thus imparting to the quartz glass a high resistance against recrystallization. In U.S. Pat. Nos. 3,370,921 and 3,472,667, elemental silicon or boron are utilized to create an oxygen deficiency in quartz bodies thus minimizing crystalline growth. And in U.S. Pat. No. 2,568,459, a glaze applied to the quartz surface retards and largely prevents quartz devitrification by preventing the diffusion of hydrogen through the hot quartz.
In the field of glass ceramics, increased resistance to breaking, cracking or failing due to mechanical impacts has been achieved according to U.S. Pat. No. 2,998,675 and 3,275,493 by combining silica, alumina and lithium oxide or magnesium oxide, in certain critical proportions, with a metal oxide crystallization catalyst. Such glass ceramic compositions are subjected to a heat treatment which results in a glass ceramic article having on its surface a thin, semicrystalline layer, which because it has a linear thermal expansion coefficient substantially lower than the interior glass, establishes a compressive stress in and parallel to the surface after the article is cooled. In other words, the interior glass shrinks more on cooling which tends to compress the surface layer in effect making it harder thus resulting in an increase in the modulus or rupture strength of the glass ceramic article. However, this phenomenon has no effect on the notoriously poor stability of glass ceramics because both the semicrystalline layer and the interior glass will rapidly devitrify and deteriorate by cracking at temperatures of 1000.degree. C. and higher as is characteristic of glass ceramics.